NASA Technical Memorandum 108533 Microgravity Processing and Photonic Applications of Organic and Polymeric Materials (MSFC Center Director's Discretionary Fund Final Report, Project No. 95-26) D.O. Frazier, B.G. Penn, D.D. Smith, and W.K. Witherow Marshall Space Flight Center • MSFC, Alabama M.S. Paley and H.A. Abdeldayem Universities Space Research Association • Huntsville, Alabama National Aeronautics and Space Administration Marshall Space Flight Center ° MSFC, Alabama 35812 March 1997 https://ntrs.nasa.gov/search.jsp?R=19970014720 2020-07-28T05:06:37+00:00Z
Transcript
NASA Technical Memorandum 108533
Microgravity Processing and PhotonicApplications of Organic and Polymeric Materials(MSFC Center Director's Discretionary Fund Final Report,
Project No. 95-26)
D.O. Frazier, B.G. Penn, D.D. Smith, and W.K. Witherow
Marshall Space Flight Center • MSFC, Alabama
M.S. Paley and H.A. Abdeldayem
Universities Space Research Association • Huntsville, Alabama
National Aeronautics and Space AdministrationMarshall Space Flight Center ° MSFC, Alabama 35812
directional coupler and their electronic analogs ................................................................. 40
V
LIST OF TABLES
° Component proportions of HITCI/Au composite ............................................................... 6
. Z (3) of vapor-deposited thin films of Pc's measured at a wavelength
of 1.9/_m 58 ......................................................................................................................... 13
° Representative absorbance intensities from scanning an approximately
1-cm-diameter spot in similar vicinities of DAMNA films formed
during vapor deposition ...................................................................................................... 28
vi
TECHNICAL MEMORANDUM
MICROGRAVITY PROCESSING AND PHOTONIC APPLICATIONS OF ORGANIC
AND POLYMERIC MATERIALS
(MSFC Center Director's Discretionary Fund Final Report, Project No. 95-26)
I. INTRODUCTION
In recent years, a great deal of interest has been directed toward the use of organic materials in
the development of high-efficiency optoelectronic and photonic devices. There is a myriad of possi-
bilities among organics which allow flexibility in the design of unique structures with a variety of
functional groups. The use of nonlinear optical (NLO) organic materials as thin film waveguides allows
full exploitation of their desirable qualities by permitting long interaction lengths and large suscepti-
bilities allowing modest power input. 1 There are several methods in use to prepare thin films such as
Langmuir-Blodgett (LB) and self-assembly techniques, 2-4 vapor deposition, 5-7 growth from sheared
solution or melt, 8 9 and melt growth between glass plates. 1° Organics have many features that make
them desirable for use in optical devices such as high second- and third-order nonlinearities, flexibility
of molecular design, and damage resistance to optical radiation. However, their use in devices has been
hindered by processing difficulties for crystals and thin films.
In this chapter, we discuss photonic and optoelectronic applications of a few organic materials
and the potential role of microgravity on processing these materials. It is of interest to note how
materials with second- and third-order NLO behavior may be improved in a diffusion-limited
environment and ways in which convection may be detrimental to these materials. We focus our
discussion on third-order materials for all-optical switching, and second-order materials for frequency
conversion and electro-optics.
A. Third-Order Materials for Optical Switching
Optical fiber communication systems have undergone stunning growth over the past decade. The
technologies that have arisen in support of these systems have been incredibly fortuitous. The operating
wavelength of erbium-doped amplifiers, for instance, serendipitously coincides with the minimum loss
wavelength of fused silica fibers. But, even as fiber optic networks have been implemented on a
universal scale, electronic switching is still the main routing method. Although fibers have dramatically
increased node-to-node network speeds, electronic switching will limit network speeds to about
50Gb/sec.Already,it is apparentthatterabit-ratespeedswill soonbeneededto accommodatethe10-15percent/monthgrowthrateof theInternetandtheincreasingdemandfor bandwidth-intensivedatasuchasdigital video.11
All-optical switchingusingNLO materialscanrelievetheescalatingproblemof bandwidthlimitationsimposedby electronics.Severalimportantlimitationsneedto beovercomesuchastheneedfor highZ (3) materials with fast response and minimum absorption (both linear and nonlinear),
development of compact laser sources, and reduction of the switching energy. The goal of minimizing
optical loss obviously depends on processing methods. For solution-based processes, such as solution
crystal growth, electrodeposition, and solution photopolymerization, it is well known that thermal and
solutal density gradients can initiate buoyancy-driven convection. Resultant fluid flows can affect
transport of material to and from growth interfaces and become manifest in the morphology and
homogeneity of the growing film or crystal. Likewise, buoyancy-driven convection can hinder
production of defect-free, high-quality crystals or films during crystal and film growth by vapor
deposition.
The guided-wave materials used most commonly have been inorganic fibers and semiconductors.
Less developed but highly promising are organic materials such as conjugated polymers, which possess
large nonlinearity with fast response times, are more easily tailored at a molecular level, and more
malleable than their inorganic counterparts. One of the major challenges for proponents of organic
materials is to find cheap, reliable methods of waveguide fabrication that take advantage of existing
technologies. Processing techniques and choices of materials should result in a minimization of both
scattering and absorption losses. One of the main reasons that organic and polymeric materials are not
more strongly competitive with silica fibers for switching applications is due to the maturity of silica
fiber processing.
B. Second-Order Materials for Electro-Optic Applications
Applications of materials with second-order nonlinearity include frequency conversion, high-
density data storage, and electro-optic modulators and switches. The first demonstration of second
harmonic generation was in quartz, 12 and it has traditionally been observed in inorganic crystals. A
decade later it was demonstrated that the second-order nonlinearity may be several orders of magnitude
larger in organic crystals possessing delocalized n-electron systems in which intramolecular charge
transfer occurs between electron donor and acceptor substituents. 13 While organic materials may offer
larger nonlinearities than inorganic crystals, the utilization of organic crystals is limited by the small
number of molecules with large hyperpolarizabilities that have a noncentrosymmetric crystalline state
(11 of the 32 crystal classes possess inversion symmetry and cannot be used as Z (2) materials).
Moreover, the maturity of inorganic crystal growth is relatively advanced while that of organic crystal
growth has not had the necessary time for comparable development.
Whereas the second-order nonlinearity in inorganic systems is a bulk effect ascribable to
crystalline structure, the primary contribution to bulk nonlinearity for organic systems is due to the
ensemble of nonlinearly responding molecules. Van der Waals forces between mers (molecular units)
2
aresmallandtheinduceddipolesresultmostdirectly from theexternalfield. This providesanaddeddegreeof flexibility for organicmaterialssincetherequiredasymmetrydoesnot requirecrystallinestructurebut insteadmaybeachievedin amorphousgeometries.For example,insteadof relyingon theart of crystalgrowth,electricfield polingof polymerscontainingthenonlinearchromophoremaybeusedto inducemacroscopicasymmetry.Alternatively,self-assemblyor liquid crystalorderingcanachievetherequiredasymmetry,possiblyencouragedin theenvironmentalquiescenceofferedbyreducedconvectionduringmicrogravityprocessing.
In order to take advantage of the large nonlinearities in organic materials resulting from
n-electron mobility while also utilizing the mechanical and thermal properties of inorganic crystals,
some researchers have turned to a semiorganic approach in which organic molecules are bound to an
inorganic host by complexation or salt formation. For example, semiorganic single crystals of
L-histidine tetrafluoroborate (HFB) have demonstrated five times the effective second-order nonlinearity
of potassium dihydrogen phosphate (KDP). 14 Single crystals are easily obtained from solution and
crystals are thermally stable with a decomposition temperature of 205 °C. Solution crystal growth has
been performed repeatedly in microgravity, particularly for the study of protein crystal growth. Many
organics, also good NLO materials, are amenable to solution growth and are ideal candidates for studies
of the kinetics and fluid dynamics of solution growth processes. Knowledge of such processes can lead
to significant improvement in crystals grown in space or on Earth. Several research tasks are underway
to study the growth of bulk single crystals of important materials such as HFB, L-arginine phosphate,
and several other organic and semiorganic molecules. There is certainly evidence that gravity could play
a role in the growth of macromolecular crystals; e.g., protein crystals, as well as on solution polymeriza-
tion processes. Rosenberger studied the temperature dependence of protein solubility, and applications to
crystallization and growth kinetics. Within these studies he observed relative interfacial kinetics and
bulk transport as functions of supersaturation and the resultant effects on the growth morphology of
lysozyme and horse serum albumin. 15 Supersaturations driven by thermal fluctuations on the order of
1-2 °C during growth lead to significant optical and structural nonuniformity in a growing crystal.
Pusey et al., using tetragonal lysozyme, showed that forced fluid flow rates of 30-40 mm/sec slow and
eventually stop the growth of 10 _tm crystals. 16 17 Cessation of growth occurred at flow rates as low as
2.5 mm/sec in some crystals, but growth persisted over a longer period of time than at higher fluid flow
rates. The conclusion is that, for some crystals at least, even small convective flows are deleterious for
growth of the crystal. The precise mechanism for this is not yet understood, but clearly the case is quite
strong for studying the growth of certain organic and semiorganic crystals in microgravity environments.
It is, therefore, reasonable to consider that microgravity can play an important role in the
formation of organic and semiorganic crystals for second-order applications. In the diffusion-limited
regime of space, larger and more defect-free crystals may be grown, hence improving optical
transparency and conversion efficiency. By different processing techniques, improved optical quality
and molecular alignment have been observed in polydiacetylene (PDA) and phthalocyanine (Pc) thin
films processed in microgravity or reduced convection environments. Perhaps of more immediate
relevance, knowledge gained from low-gravity (low-g) experiments may enable the optimization of
growth conditions on Earth.
C. Material Limitations
1. Dispersion and Loss
One limitation governing the propagation of pulses in a waveguide is the reshaping of the pulse
due to group velocity dispersion (GVD). Each frequency component in the pulse sees a different
refractive index and thus travels at a different speed through the medium, leading to a spreading or
reshaping of the pulse. GVD is more important for short pulses since they contain more frequency
components. This pulse spreading reduces the amount of information that can be carried and destroys
the cascadability (the ability of the output of a device to reenter an identical device and behave
similarly) of the system. However, a pulse also consists of different intensities. Each intensity can also
see a different refractive index, due to the nonlinearity of the medium, and so travels at a different speed.
The ability of a pulse to become reshaped due to the nonlinearity of the medium is known as self-phase
modulation (SPM). For certain pulse shapes, GVD is exactly balanced by SPM and the pulse retains its
shape over an infinite propagation distance in the absence of loss. These pulse shapes are known as
solitons. In practice, however, solitons cannot propagate infinitely. The effect of dissipation decreases
the peak power in the pulse, hence decreasing SPM. The result is that the soliton widens as it propagates.
Two main sources of loss are operative in silica fibers--an intrinsic loss due to Rayleigh scattering and,
superimposed on this, absorption due to hydroxide ion impurities in the melt. 18 Due to the maturity of
silica fiber fabrication technology, impurities in the melt can be kept quite low. Microgravity processing
may enable polymeric materials to become more competitive with silica fibers through the reduction of
similar types of scattering loss mechanisms.
2. Two-Photon Absorption
A major setback in the development of an all-optical switch is the existence of nonlinear loss
manifested by multiphoton absorption. The current strategy is to search for a truly nonresonant operating
wavelength. To demonstrate the deleterious effects of multiphoton absorption, consider its effect on a
simple device. A nonlinear directional coupler (NLDC) is a device in which two waveguides or fibers
are brought close together such that their overlapping evanescent attenuations periodically transfer
power back and forth between the guides as light propagates through the device. 19 The periodicity of
phase-matched power transfer depends on channel separation and refractive index. Full power transfer
occurs after one coupling length, Lc. When the length of the overlapping region equals the coupling
length, all the power will be transferred to the neighboring guide. If the guide material has a large cubic
nonlinearity (and hence, nonlinear refractive index, n2), the coupling will be perturbed at high intensities
due to an intensity-dependent phase mismatch in the guides, and at the switching power, the majority of
the transmission will switch from one guide to the other. The majority of the coupler's output may then
be toggled between the guides simply by raising and lowering the intensity.
Multiphoton absorption occurs when the intensity of the radiation causes virtual levels to become
accessible. In centrosymmetric media, transition rules prohibit transitions between same parity states
unless virtual levels are involved. Because two-photon absorption (TPA) is intensity-dependent, it varies
with propagation distance. The result is that power transfer is accompanied by increasing TPA in the low
4
intensity guide and decreasing TPA in the high intensity guide such that complete switching does not
occur or is suppressed (fig. 1). Mizrahi et al. 2° have shown that TPA places a fundamental constraint on
the usefulness of any high Z (3) material. Semiconductors, for example, are constrained to energies below
half the band gap to avoid TPA. 21 22
1.0
i
Z
0.0
TPA
Ps
InputPower
FIGURE 1.-- Effect of TPA on an NLDC. The coupler is initially in a
crossed state. Increasing the input power perturbs the
coupling, resulting in switching, but the presence of the
TPA suppresses switching.
An alternative approach based on materials architecture is also possible for eliminating induced
absorption. Specifically, the nonlinear absorption in a material may be canceled by the addition of small
metal particles. This approach is possible because of a counterintuitive consequence of local field
effects that was f'trst recognized by Hache et al.23 They found that for gold nanoparticles in glass, the
colloid as a whole demonstrated saturable absorption. However, the metal itself behaved quite
differently, demonstrating induced absorption. Embedding the particles in a glassy matrix altered the
sign of the nonlinear absorption as a result of the local field correction. The implication of Hache's
finding is that there is a concentration somewhere between pure gold and the colloidal gold glass at
which the imaginary part of the cubic susceptibility goes to zero. In fact, if the sign of lmz (3) is the
same for each component, then by necessity there will actually be two concentrations at which
lrnz (3) = 0. The smaller concentration crossing point is obviously more useful since it entails a lower
amount of linear absorption and poses less of a challenge to fabricate. The significance and specificity
of particle concentration demands careful control of metal particle dispersion. This requirement is also
prevalent in crystal growth of certain semiconducting alloys where the optimum band gap is determined
by alloy stoichiometry. Lehoczky et al. directionally solidified Hgo.84Zno. 16Te alloy in microgravity
with some success at purely diffusion-controlled growth to achieve the desired stoichiometry throughout
thespace-growncrystal.However,unexpectedtransverseresidualaccelerationsfrom the spaceshuttleenvironmentresultedin someradialcompositionalvariations.24Theresultstressesthedifficulty inachievingpreciseconcentrationsduringmaterialsprocessingwherethermalor concentrationgradientsarise,andcouldbe fatally disruptivein termsof achievingadefinedconcentrationobjective.Consideringdifficulties in achievingdiffusion-controlledgrowth,andotherdesirablegravitationallysensitiveresultsduringmicrogravityprocessing,ground-basedattemptsat overcomingtheseeffectsmayprovefutile. Wherehomogeneousdispersionandconcentrationarerequisitein materialsofinterest,processingon Earth-orbitingplatformscouldoffer distinctadvantages.
To demonstrate the cancellation of photoinduced absorption in a composite system, Smith 25
performed open aperture z-scan measurements on a gold colloid prepared by the recipe of Turkevich 26
at various particle concentrations. A frequency-doubled Nd:YAG laser at 532 nm provided 30-ps,
mode-locked pulses at a repetition rate of 10 Hz. Each concentration was placed in a 1-cm optical
pathlength quartz cuvette on a track near the focus of the beam. The focal length of the lens was 33 cm
and the full width half maximum (FWHM) beam diameter was 2.5 mm. The beam waist was measured
to be w0 = 70/.tm, corresponding to a Rayleigh diffraction length of z0 = 2.9 cm.
Instead of TPA, however, the nonlinear mechanism was reverse saturable absorption. The system
consisted of a known reverse saturable absorber 1,1', 3, 3, 3', 3' -hexamethylindotricarbocyanine iodine
(HITCI) in methanol and water. The proportions of dye, methanol, and water were held fixed. Since
these chemicals form a solution, they may be considered as a single component, although their chemical
association should be accounted for by a modification of the local field factor as described by
Frrhlich. 27 The other component was the gold. The proportions of the various components are
illustrated in table 1. Note that the same amount of dye, methanol, and water is used in each case and
that the only variable is the concentration of gold. The z-scans for various concentrations of gold are
displayed in figure 2. It can clearly be seen that the nonlinear absorption changes sign near curve 6.
The experiment was repeated several times and the sign reversal was obvious each time. Note that
curves 1-5 display a valley indicative of reverse saturable absorption. At the focus, however, a small
secondary peak is observed. This peak corresponds to a saturation of the nonlinear absorption and was
first reported by Swatton et al. 28 A four-level semiclassical model was used to describe the absorption
of the dye.
TABLE 1.--Component proportions of HITCI/Au composite.
using the radiation from an argon ion laser: (a) demonstration pattern
on a quartz disk, and (b) enlarged image of an actual Mach-Zehnder
interferometer on a glass microscope slide.
9
It has been proposed that NLO thin-film properties may be improved by low-g processing using
electrodeposition. Strong candidates for NLO thin-film applications are the polythiophenes (pT).
Polymeric thiophenes are attractive materials due to their ease of preparation, stability, and high
third-order susceptabilities. 48-52 A simple and convenient method for preparation of pT's is electro-
chemical oxidation. Earlier microgravity experimentation 53 54 on metal, metal/cermet electrodeposition
of Ni provides some microgravity electrodeposition background and raises the possibility of application
to improving the quality of pT thin films in low g. Electrodeposition of Ni on an Au substrate in low g
often results in the production of an x-ray nondiffracting surface. Similarly deposited cobalt metal does
not give this result nor does Ni on a glassy carbon substrate. Further, Co/Ni alloy composition variance
during electrodeposition is strongly dependent upon the amount of convection. Similar sensitivities to
gravitational influences apparent in inorganics during electrodeposition should arise during
electrodeposition of organics.
Electrochemical polymerization deserves thorough investigation for use in fabricating thin film
waveguides. NLO films might be prepared on the surface of various substrates during polymerization
for the fabrication of devices. This method has been useful in the synthesis of several polymers, in
addition to the pT's, such as polypyrrole and polyazulene. The probable existence of thermal and
concentration gradients in such dynamic processes suggests an assessment of gravitational influences
on film morphologies is justified.
10
II. GROWTH OF THIN FILMS BY VAPOR DEPOSITION
A. Phthalocyanine Thin Films
This class of materials is an excellent candidate for use in developing NLO devices because
of their two-dimensional planar n-conjugation, better chemical and thermal stability than most other
organic materials, and ease of derivatizing through peripheral and axial positions (fig. 4). Large 55-64
and ultrafast 52 65 66 third-order nonlinearities have been demonstrated for Pc's. The nature of the
central metal atom strongly influences the value of Z(3). Shirk et al. 56 measured the third-order
susceptibility of tetrakis (cumyl phenoxy) Pc's by degenerate four-wave mixing at 1.064/.tm. The
X(3)xxxx values for Pt-Pc (2 x 10 -1° esu), lead phtalocyanine (Pb-Pc) (2 × 10-11 esu) were found to be
approximately 45 and 5 times that of the metal-free Pc form (4x 10 -12 esu), respectively. Other
researchers have shown that Z (3) of Pc's increase about 15 times by changing the central metal atom
from silicon to vanadium 67 and there is also an increase when the central atom is a heavy metal atom
such as lead. 68 These positive influences of metal substitution on the third-order optical nonlinearity
are attributed to the introduction of low-lying energy states derived from metal-to-ligand and ligand-to-metal charge transfer. 56
_N
N
N-- --N
FIGURE 4.--Metal-free Pc (H2Pc).
11
Recently,Pc-relatedcompoundshaveemergedasnovelcandidatesfor second-orderNLO applications.Sastreet al.69measureda fl value of 2,000 x 10 -30 for the trinitro-substituted boron subphthalocyanine
(subPc) whose structure is shown in figure 5. SubPc's are two-dimensional,
14-_r-electron conjugated macrocycles that are composed of three isoindole units containing boron
inside. The reported second-order hyperpolarizability for the trinitro derivative is comparable to that
of the most efficient linear or dipolar compound.
R
R R
R = NO2, or H, or tert-Bu
FIGURE 5.--Trinitro-substituted boron subPc.
Difunctional tetraarylporphyrins with nitro groups as electron acceptors and amino groups as
electron donors were observed to have a substantial hyperpolarizability by Suslick et al. 70 The fl values
for cis diamino-dinitro and triamino-mononitro substituted tetraarylporphyrins were 30 × 10 -30 esu and
20 x 10 -30 esu, respectively. These derivatives were prepared by the partial reduction of the nitro groups
of 5, 10, 15, and 20 tetrakis-(p-nitrophenyl) porphyrin. Li et al. 7] measured a relatively large second
Wada et al. 72 measured a Z (3) of 1.85x 10 -10 esu at 1.907 pm for a 51.4-nm-thick film of
VOPc vacuum deposited onto quartz. The %(3) values for two different phases in vanadyl and titanyl
phthalocyanines (TiOPc's) were measured by optical third-harmonic generation at wavelengths of
1,543 nm and 1,907 nm by Hosoda et al.73 The transformation of as-prepared Pc films from phase I to
phase II was performed by thermal annealing and was accompanied by a red shift in absorption spectra
and an increase in %(3) values of 2-3 times. % (3) values for as-prepared films of VOPc and TiOPc were
3.8x 10 -ll esu and l0 -11 esu, while the annealed films had values of 8.1 x 10-11 esu and 4.6x 10 -ll esu,
respectively.
Recently, relatively strong second-harmonic generation (SHG) was reported for vacuum-
deposited CuPc films which possess inversion symmetry. Chollet et al. 74 measured a deft = 2 x 10 -19
esu at 1.064/_m fundamental wavelength for films with thicknesses ranging from 50 to 500 nm that
were prepared at a pressure of 10 -6 ton- and source temperature of 120 °C. The films were homoge-
neous and partly oriented with a relatively large distribution of molecular axes, oriented almost
perpendicular to the substrate. This order was confirmed by SHG measurements. SHG in the films was
attributed to quadrupolar or dipolar origins. Kumagai et al. 75 prepared 40- to 2,000-_-thick films at a
pressure of 5 x 10 -6 ton. and obtained Xzyy = 4 x 10 -8 esu at 1.06/lm fundamental wavelength, which
is a quarter of the value for LiNbO3. They proposed a mechanism in which an asymmetric crystal field
acting perpendicular to the surface makes each CuPc molecule capable of SHG. Yamada et al. 76
performed in situ observation of SHG from CuPc films during vacuum evaporation on glass at a
pressure of 1.5 x 10 -5 ton. and a rate of 1.2 nm/min. Thickness dependence on SHG was compared with
calculations based on electric dipole, electric quadrupole, and magnetic dipole mechanism to clarify
the origin of SHG activity. Based on the comparison, SHG was ascribed to electric quadrupolar or,
preferably, magnetic dipolar origin.
13
Theresearchof Debeet al.77-79indicatesthatbetterqualityorganicthin films for usein NLOdevicesmightbeobtainedby closed-cellPVT in microgravity.In thePVT process,thesourcematerialissublimedin aninert gasandallowedto convector diffusedownathermalgradientandto ultimatelycondenseat acrystalor thin film growthinterface.77Theadvantageof thin film growthin microgravityis thatit providestheopportunityto eliminatebuoyancy-drivenconvection.Recentreports77-79of thespaceshuttlemissionSTS-51of August/September1985includeresultsof experimentsin whichCuPcwasepitaxiallydeposited,by PVT,ontohighlyorientedseedfilms of metal-freePc(HzPc).Thesubstratewasa 1.4-cm-diametersolidcopperdisc.
ThePVT of organicsolidsapparatususedto grow CuPcthin films consistedof nineidenticalmetal/Pyrexampouleshousedwithin itsown heaterassemblyandvacuuminsulationcell.77788oFilmsweregrownin 1.7-cm-diameterby 7.5-cm-longPyrextubesthatwereplacedwithin resistancewire-woundheaterswhich inducedanonlinearaxial thermalgradient.Thegrowth ampouleswerefilledprimarily with CO2,H2,thebuffergasXe,or He andtheneitherN2or CO asthenextmostabundantcomponent.A computer-controlledheatermaintainedthehotendof theampoule,containingthesource,at400 °Cfor 4 hoursafterthecruisetemperaturewasreached.Thesubstratewasmaintainedatatemperatureof 70 °C by aheatpipe.
Thesubstrateseedfilm waspreparedby vacuumsublimationof H2Pcontoacopperdiscat atemperaturerangeknownto producehighlyorientedfilms. A metal-freePcfilm with athicknessof1,100+ 50 ,_ was grown at a deposition rate of 70 _dmin. The substrate was held at a temperature of
5 to 10 °C and the source-to-substrate distance was 16 cm. Debe has shown that highly oriented films
of H2Pc are obtained when the substrate temperature range is approximately 5 + 5 °C. 8° Before starting
the growth process, the source material was outgassed by slowly increasing the temperature of the hot
zone. After - 2 hours of "soaking," the temperature was increased to cause the H2Pc material to deposit
on the copper substrate at a pressure of about 5 × 10q5 torr.
Microgravity-grown CuPc films had several desirable features which indicate that the growth
of organic films in low-g may result in better quality films for NLO applications. For example, results
of analysis by visual photography, bright field and differential interference contrast microscopy,
scanning ellipsometry, visible reflection spectroscopy, and direct interferometric phase contrast
microscopy imply that the space-grown films were radially more uniform and homogeneous, and an
order of magnitude smoother over the submillimeter to submicron scale range. 77 Results of analysis
involving the use of external reflection-absorption infrared (IR) spectroscopy, grazing incidence x-ray
diffraction, and visible-near IR refection-absorption spectroscopy infer that the microgravity-grown
films are more highly uniaxially oriented and the films were found to consist predominantly of
crystalline domains of a previously unknown polymorphic form of CuPc. 78 In addition, scanning
electron microscopy analysis revealed that there was a distinctly different microstructure in the center
of the space-grown films and that the circular perimeters of the microgravity-grown films had
microstructure much like that of the ground control films in both their center and edge regions. 79
As stated earlier in the chapter, electric field poring of polymers containing the nonlinear
chromophore may be used to induce macroscopic asymmetry. However, in the case of the ring-
14
structuredmacromolecule,phthalocyanine,poling is notgenerallyanoption to achieve ordering in
deposited films. It is therefore beneficial to achieve, if possible, self-assembly to induce required
asymmetry by other means. Whenever self-assembly might occur in molecules not prone to poling,
exploitation of conditions favorable toward asymmetry could prove beneficial. In the case of Pc, for
example, X (3) enhancement and possible X (2), inducement might result from self-assembly. Vapor-
deposited H2Pc have demonstrated some potentially interesting NLO properties. Researchers report
these films to be randomly oriented when processing occurs in 1-g (fig. 6a). 43 44 77-80 From
microgravity processing, CuPc films epitaxially deposited onto H2Pc films are highly oriented and
densely packed (fig. 6b). Abdeldayem et al. 81 recently observed intrinsic optical bistability in vapor-
deposited thin films of metal-free Pc, ranging in thickness from 40 to 800 nm, using continuous wave
(CW) and chopped He-Ne lasers at 633 nm. Source and substrate temperatures were maintained at
300 °C and 5 °C, respectively, while vapor vacuum deposition occurred at 10 -6 torr onto quartz disks.
Bistability in the film was attributed to changes in the level of absorption and refractive index caused
by thermal excitation. This nonlinear effect could improve dramatically in highly oriented microgravityprocessed films.
(a)
_t-g
30,O00X
(b)
1-g
0 45° View
FIGURE 6.--CuPc films epitaxially vapor-deposited onto copper substrate: (a) #-g deposition of
CuPc epilayer, and (b) 1-g deposition of CuPc epilayer (courtesy of 3M Corporation).
15
Foradiscussionof opticalbistability,we first recognizethattheabsorptionspectrumin thevisibleregionshowsa strongmaximumat 626nm.In oneexperiment,81theCW He-Ne(632.8nm) laserat afixed input powerof ~30mW wasfocusedona 230-nm-thickfilm. Thebeamtransmissionincreasedtemporallyoveraperiodof nearly12hoursasshownin figure 7a.Thenearlystraightline of figure 7bis afractionof the input powerfor monitoringlaserstability.Fitting thetransmissiondatato a singleexponential(thesolid line) gavearise timeof -2.2 hoursto reacha steadystate.
m
t_
Egg
I,,-
1.4
1.2
1.0
0.8
0.6
0.4
0.2
J- //
D //,
I
/ i __ _ _ti7_ "_-tw'a"_--_-'_e-_
___/_ _. (a)
_._"
J'
(b)
I L I I I I I0 2 4 6 8 10 12 14
Time(hours)
FIGURE 7.--(a) Time-dependent transmittance through a metal-free Pc film (230-nm-thick),
using a CW He-Ne laser at 632.8 nm and 30 mW power. The dotted curve
represents the experimental data, while the solid line represents a single
exponential theoretical fit. (b) The dotted line represents laser stability throughout
the experiment, while the solid line is the corresponding straight-line fit.
The temporal transmission effect in the film can be explained by the following sequence: (a) The
initial low transmission of the beam through the film can be attributed to a strong absorption of the beam
which generates free electrons and holes; (b) these free charges relax to excitonic states 82 and release
their excess energy as heat to the system at the focal point; (c) the build-up of localized heating at the
focus reduces absorption of the He-Ne radiation, causing saturation of absorption. This factor is in
16
agreement with the results of a separate experiment to measure the sample's absorption at a temperature
elevated above room temperature by a water bath. Such reduction in absorption allows moretransmission to occur with time.
Investigation of the bistability of metal-free Pc films of 833-nm thickness used a chopped
He-Ne 632.8-nm laser beam at frequencies ranging from 100 to 750 Hz. The film was positioned on a
micrometer stage, at the lens focus, and transversely translated in and out of the beam alternately to
record intensity input and film transmittance. A Hewlett Packard (HP) digitizing oscilloscope, model
54120B, recorded the input and the transmitted pulse with an HP plotter, model 7470A (fig. 8). The
nonsymmetrical shape of the transmitted pulses (fig. 8b) indicated the presence of intrinsic bistability in
metal-free Pc. Figure 8c depicts typical bistable switching, constructed from the transmitted pulse. The
switching power of ~ 0.33 mW per pulse in combination with a pulse duration of 1.37-ms recovery time
yields a very low switching energy of - 0.45 nJ. Observation of bistability was repetitive in the same
film using a CW He-Ne laser as shown in figure 9 for different timespans between successive points.
I
o
(c)
b
h
D
D
R
B
q
m
I I I 1 I I I1 2 3 4 5 6 7
Input (au)
FIGURE 8.--The bistability loop of an 833-nm metal-free Pc film using a chopped
CW He-Ne laser at 632.8 nm: (a) the input pulse, (b) the transmitted
pulse, and (c) hysteresis switching constructed from (b).
17
I-
O
2.5
2
1.5-
1
0.5-
Oi0.1
(a)
I0.2 0.3 0.4
Input Intensity (au)
i °40
0,5 0
(b)
0.4 0.8 1.2 1.6
Input Intensity(au)
m
cm
2
= 1
00.2
(c)
0.4 0.6
Input Intensity(au)
(d)
6
d_
4
.._52
00.8 0.2 1.20.4 0.6 0.8 1.0
InputIntensity(au)
FIGURE 9.--The bistability loops for different timespans of a metal-free Pc film of 232.5-nm
thickness using a CW He-Ne laser. Timespans between successive points are
(a) 3.84 sec, (b) 10 sec, (c) 342 sec, and (d) 1,800 sec. (a) Shows the least
prominent bistability loop while (b), (c), and (d) show a minimal effect of the
timespan between points.
A thinner film, 230 nm, also demonstrated saturation of absorption at the same He-Ne laser
frequency. Figure 10 illustrates the experimental data recording absorption by a 230-nm-thick film and
the theoretical curve-fit assuming Bloch-type saturable absorption with negligible scattering losses 83
a(1)L = [aoLY(1 + 1/Is)] (1)
where Is is the threshold power of the saturation, L is the thickness of the sample, and ao is the linear
absorption coefficient. The saturation intensity, estimated from the theoretical fit, is
Is - 2.0 x 10 4 W/cm 2 .
Modeling Pc as a three-level system, a molecule in the ground state at saturation absorbs light
at a rate
lit = So Is / hf (2)
18
3.8
4I t_" 3.0
• _I'
_ 2.6
2.2
1.81.0x104 1.2x104 1.4x104 1.6x104 1.8x104
InputIntesity(W/cm2)
FIGURE 10.--Experimental and theoretical fitting of saturable absorption of a metal-
free Pc film (232.5-nm thickness) at 632.8 nm from a CW He-Ne
laser. The corresponding saturation intensity is 19.8 x 103 W/cm 2.
where t is the decay time of the excited triplet state, Is is the saturation intensity, So is the absorption
cross section of the groundstate, and hfis the energy of the incident photon. From the measurements
of the fall time of 1.0756 ms at 245 Hz in figure 6b, the absorption cross section was estimated 84to be on the order of -2.4 x 10 -17 cm 2.
The estimated third-order nonlinear susceptibility measurements by four-wave mixing using
pulsed Nd:YAG laser at 532 nm was on the order of 10 -8 esu. This relatively large value is attributed
to both resonant as well as thermal mechanisms that might be present in the system at this wavelength.
B. Polydiacetylene Thin Films
1. Second and Third-Order NLO Properties of PDA's
PDA's (fig. 11) are highly conjugated organic polymers that are of considerable interest because
of their unique chemical, optical, and electronic properties. 85-88 This class of polymers has received
extensive attention as organic conductors and semiconductors, as well as NLO materials. The high
mobility of the n-electrons in the polymer backbone allows them to have large optical/electrical
susceptibilities with fast response times. They can be highly ordered, even crystalline, which is
important for optimizing their electronic and optical properties, and they can readily be formed into thin
films, which is the preferred form for many applications. The physical, chemical, and mechanical
19
% /C-- C_ C_ C
/ %x
Y
FIGURE 11 .--Structure of PDA repeat unit.
properties of PDA's can be varied by changing the functionality of the side groups, thereby making it
possible to tailor their properties to meet specific needs. Thus, there is a great deal of interest in the use
of these polymers for technological applications.
PDA's are among the best known third-order NLO materials, and there has been considerable
investigation over the past 20 years into their properties. Single crystals of poly (2,4-hexadiyne-1,6-
ditosylate), also known as PTS, possess one of the largest (possibly the largest) nonresonant third-order
optical nonlinearities ever measured, on the order of 10 -9 esu. 89 The third-order NLO properties of
numerous other PDA crystals and thin films have also been determined. Typical Z (3) values for PDA's
range from 10 -7 to 10 -12 esu, depending on the degree of resonance enhancement and other factors.
Both theoretical and experimental results have determined that the Z (3) value is approximately 100 times
greater along the PDA backbone than perpendicular to it, demonstrating the effect of the conjugated
7r-electron system. Because the origin of the nonlinearity is electronic, they can have very fast response
times, on the order of femtoseconds.
More recently, PDA's have been investigated as potential second-order NLO materials.
Theoretical calculations have indicated that certain PDA's could possess extremely high second-order
NLO susceptibilities; e.g., molecular hyperpolarizabilities on the order of 1,000 x 10 -30 esu. 90 In order
to make use of this second-order nonlinearity, it is necessary to orient the polymers into acentric
structures, either crystals or thin films. This is not trivial; many compounds which have desirable
properties at the molecular level tend to orient themselves centrosymetrically in the bulk to
minimize electrostatic interactions.
SHG has been observed from certain asymmetrical liquid-crystalline diacetylene monomers
(although, interestingly, not from the corresponding polymers) 91 from both LB and self-assembled PDA
monolayer and multilayers 92 93 and even from a spin-coated PDA film. 94 Lastly, powder SHG
efficiencies comparable to that of 2-methyl-4-nitroaniline (MNA) have been obtained from vapor-
deposited polycrystalline films of a PDA possessing MNA as a side group. 9° If the crystallites are
partially aligned by growing the films quasi-epitaxially onto prealigned poly (tetrafluoroethylene)
substrates, the SHG efficiency increases by almost one order of magnitude.
2. Potential Benefits of Microgravity Processing
Optical applications require the formation of high quality thin PDA films; i.e., films possessing
minimal defects such as impurities, inhomogeneities, light-scattering centers, and so forth. The standard
20
techniques for obtaining PDA thin films involve the growth of crystalline diacetylene monomer films or
the deposition of LB films, followed by topochemical polymerization of these films in the solid state to
yield ordered PDA films. This ability to undergo solid-state polymerization is a very intriguing property
of diacetylenes; in principle, one can start with a single crystal monomer and obtain a single crystal
polymer.7 95 However, the process is not trivial; the formation of high-quality, crystalline diacetylene
monomer films or LB films can be very tedious and difficult, and, furthermore, by no means do all
monomers polymerize readily in the solid state. 8 Achieving high-quality PDA films requires the growth
of high quality diacetylene monomer films, which are then topochemically polymerized. A commonly
employed technique for obtaining diacetylene monomer films is vapor deposition. One of the chief
limitations to vapor deposition of high quality monomer films; e.g, single crystalline films with good
molecular orientation and few defects, has been a lack of understanding of how the processing
conditions affect monomer film growth. It is certainly well known that parameters such as temperature,
pressure, concentration, and so forth can affect vapor transport processes. One parameter which is often
tacitly ignored is the influence of gravity. However, the effects of gravity, such as buoyancy-driven
convection, can greatly influence heat and mass transport during the growth process, and thereby
influence all of the aforementioned growth parameters. Thus discerning the effects of gravity (or the
lack thereof) could play a critical role in optimizing the growth of high-quality PDA films by vapor
deposition.
One method of assessing convection in a gas phase is to perform the computation at low pressure
to relieve the need for specific materials constants. It is important to note that buoyancy effects are
possible only if the molecular mean free path is short enough relative to cell dimensions such that
molecular flows are not in the free molecular flow regime. A mathematical model has been developed
to determine buoyancy-driven heat transfer in an ideal gas under a variety of orientations relative to
gravitational accelerations. 96 The model demonstrates that convection can occur at total pressures as
low as 10-2 mm Hg in cells having relatively high length-to-width ratios. A preliminary experimental
test of the model involved deposition of the diacetylene monomer, 6-(2-methyl-4-nitroanilino)-2,
4 hexadiyn-l-ol (DAMNA) (fig. 12), at an evacuation pressure of 10 -2 mm Hg.
CH 3
HOCH2 _ C _ C _ C _ C _ CH2 _ NH -_///("_% NO2
FIGURE 12.--Diacetylene monomer, DAMNA.
The deposition of DAMNA by PVT was of 30-min duration. Cell dimensions were the same
as those depicted in figure 13 with source temperature of 120 °C and sink temperature of 30 °C. The
evacuation pressure reached a minimum in the presence of DAMNA (10 -2 mm Hg), and was signifi-
cant/y lower in the absence of DAMNA. It is probable that the DAMNA vapor pressure is equal to,
or greater than, the evacuation pressure of 10-2 mm Hg. From a different study, the measured vapor
pressure of 4-N,N-dimethylamino-4"-nitrostilbene (DANS) at 120 °C was reportedly 0.374 ram. 97
21
(.DoO£M
d_
T= 120°C
?
ii
Z
J Sin(
t
R I
I
L 2
L I
, R2_
T= 120°C Source R2
o_oJ,m-
II
, /"
I
(a) (b)
FIGURE 13.--Vapor deposition cell for PVT of DAMNA.
This is a relatively large organic molecule having a molecular weight of 211 g/mole as compared to
DAMNA with a molecular weight of 247 g/mole. Considering structural and size similarities, we may
Recently, a novel process has been discovered for the formation of thin amorphous PDA films
using photodeposition from monomer solutions onto transparent substrates. 33 45 46 90 Specifically,
polymeric fills were directly synthesized from a diacetylene monomer, DAMNA (fig. 12), derived
from 2-methyl-4-nitroaniline (MNA) that only sluggishly polymerizes when the crystalline monomer
is irradiated.l°° This compound was one of several asymmetric diacetylenes that were first studied
extensively for their optical and electronic properties by Garito and coworkers in the late 1970's;
however, their investigations did not include behavior in solution. 101 102 It was found that thin
polymerized DAMNA (PDAMNA) films can be obtained readily from solutions of DAMNA in
1,2-dichloroethane by irradiation with long wavelength UV light through a quartz or glass window,
which serves as the substrate. This simple straightforward process yields transparent films with
thickness on the order of a micrometer.
Despite the considerable volume of literature available on diacetylenes and PDA's, this solution-
state photodeposition reaction has never been reported. Thus many of the parameters controlling the
efficacy of the process are not yet known. The basic idea is quite straightforward; the diacetylene
monomer solution is irradiated through a UV transparent substrate and a thin PDA film results. To date,
several diacetylene monomers have been tested and found to be capable of photodeposition of polymeric
films from solution.
Special chambers were Constructed for carrying out the reaction and obtaining thin films on
small round substrate disks (fig. 18). To obtain a PDAMNA thin film, a solution of DAMNA in
1,2-dichloroethane (approximately 2.5 mg/ml = 0.01 moles/L) is placed inside the chamber, and the
chamber is then irradiated through the substrate with long wavelength UV light (365 nm). The thickness
of the resulting PDAMNA fill depends on the duration of exposure and the intensity of the UV source.
After the photodeposition is complete, the monomer solution, which now also contains suspended
particles of precipitated polymer, is removed from the growth chamber. The substrate, now coated
on one side with the PDAMNA film, is then removed, washed with 1,2-dichloroethane, and dried.
Thin PDAMNA films obtained in this manner are transparent, glassy yellow-orange in
appearance, suggesting an amorphous nature. Both refractive index measurements and electron beam
diffraction studies indeed indicate that the films are amorphous. The fills are insoluble in organic
solvents, although solvents such as acetone can cause them to peel off of the substrate. Even con-
centrated sulfuric acid does not dissolve the films; they turn brown, shrivel, and peel off of the substrate,
but do not dissolve, even after several weeks. In contrast, the PDAMNA powder precipitated from the
bulk solution is soluble in solvents such as acetone and dimethylsulfoxide (DMSO).
29
MonomerSolution
Substrate
_ uv
Thin Film
PolydiacetyleneThinFilmGrowthChamber
MaskedPDAMNAFilmonGlau
(a) (b)
FIGURE 18.--(a) PDA thin film growth chamber, and (b) masked PDAMNA film on glass.
The exact role that the substrate plays in photodeposition of PDA films from solution is not yet
fully understood. Apparently, any substrate which is sufficiently transparent to UV light can be used;
thus far we have grown PDAMNA films onto glass, quartz, mica, indium-fin oxide-coated glass,
polyethylene teraphthalate, KBr, and NaC1. In order to gain some insight into the process that occurs
at the surface of the substrate during photodeposition, masking experiments were conducted in which
a portion of the substrate is blocked from exposure to the UV light during film deposition. The mask is
placed on the exterior surface of the substrate (opposite the side on which the film is grown), and thus
is not in contact with the solution; it serves merely to protect part of the substrate from the light.
Interestingly, the result is that film deposition occurs only where the substrate is directly exposed to the
light (see figure above); absolutely no film deposition occurs behind the mask, even though polymeri-
zation takes place throughout the bulk solution. This clearly indicates that polymerization is occurring
at (or very near) the surface. If this were simply a case of bulk solution polymerization, followed by
adsorption of the polymer onto the substrate, the mask, because it is on the outside, should have no
effect; film deposition would be expected to occur over the entire substrate.
2. Third-Order NLO Properties of Films
Based on optical microscopy, and on refractive index measurements using waveguide mode
analysis, PDAMNA thin films obtained via photodeposition from solution have good optical quality,
superior to that of films grown using conventional crystal growth techniques. Considering the simplicity
of photodeposition, this technique could make the production of PDA thin films for applications such
as NLO devices technologically feasible. Hence the NLO properties of the PDAMNA films were
investigated, specifically, their third-order NLO susceptibilities.
Degenerate four-wave mixing experiments carried out at 532 nm on PDAMNA films obtained
by photodeposition from solution (thickness around 1.0/tm) yield 2'(3) values on the order of 10 -8 to
3O
10-7 esu.Qualitativemeasurementsindicatethattheresponsetimeis on theorderof picoseconds,whichis consistentwith anelectronicmechanismfor thenonlinearity.It shouldbepointedout that because532nm is in theabsorptionedgeof thepolymer,theZ (3) values obtained are resonance-enhanced.
Typically, Z (3) values for PDA's can vary by several orders of magnitude, depending on the degree of
resonance enhancement and other factors. 87 The largest reported nonresonant (purely electronic) 2'(3)
value for a PDA is on the order of 10 -9- 10 -1° esu for PTS single crystals. 1°3 In order to obtain a valid
measure of the inherent nonlinearity of the PDAMNA films, experiments need to be conducted at longer
wavelengths where the polymer does not absorb (in the case of PDAMNA, above 700 nm). Preliminary
measurements with the PDAMNA films using a Ti-Sapphire laser at 810 nm give 2"(3) values on the
order of 10 -11 esu, with response times on the order of femtoseconds. 104 There are no indications of
either one- or two-photon absorption at this wavelength. To ascertain the true potential for device
applications (the figures of merit), thorough measurements of light scattering, linear absorption, two-
and three-photon absorption, damage thresholds, and so forth must be carded out. 1°5 Such experiments
are underway and will be the subject of future publications.
Thus far, these films have not been studied for second-order NLO properties because they are
amorphous; although studies (atomic force microscopy, scanning electron microscopy, and UV-visible
spectroscopy) do indicate that there is partial chain alignment in the direction normal to the substrate.
At present, the degree of orientation is too low to exploit any potential second-order nonlinearity.
However, it may be possible to improve the orientation in the films by means such as electric field
poling, surface modification of the substrate, or even modifying the polymer structure (e.g., attaching
a liquid crystal moeity). Ordered PDA films would not only be capable of second-order nonlinearity,
but should also exhibit increased third-order nonlinearity. Additionally, electronic applications such as
one-dimensional conductors require films with aligned polymer chains.
3. Fluid Dynamic Analysis
It is well-known that gravitational effects, such as buoyancy-driven convection, can affect heat
and mass transport processes in solution. 106 Photodeposition of PDA films from solution is no
exception. We shall first discuss how buoyancy-driven convection can arise during photodeposition of
PDAMNA films from solution, and then describe how this convection can affect the morphology,
microstructure, and properties of the films obtained. Both the monomer solution and the film generate
heat due to absorption of UV radiation. The radiative heating, along with the thermal boundary
conditions of the walls of the thin film growth chamber, will give rise to a complex temperature pattern
in the solution. Due to the lack of thermodynamic equilibrium, the solution will possess temperature and
concentration gradients, and therefore density gradients. These gradients, under the influence of gravity,
can induce convective fluid flows in the solution (buoyancy-driven convection).
The onset of thermal convection is determined by a stability parameter known as the Rayleigh
number, Ra, defined as 106
Ra - ot g d 3 AT , (4)VK
31
whereo_is the coefficient of thermal expansion of the solution; g is the acceleration due to gravity; AT is
the temperature difference across distance, d, in the solution; v is the kinematic viscosity; and tcis the
thermal diffusivity. For photodeposition of PDAMNA films, the value of AT (over a distance of less than
1 mm) can vary from a only a few tenths of a degree to several degrees, depending on the intensity of
the UV radiation. In order to grow thicker films (> 1/.t), higher intensity radiation is necessary, making
large temperature gradients unavoidable. The intensity and flow pattern of convection can be predicted
when the Rayleigh number is known. For instance, for an infinite fluid layer in the horizontal direction
with a temperature gradient in the vertical direction (co-linear with gravity), convective motion will
occur in the form of rolls with axes aligned horizontal when Ra > 1,708 (the critical Rayleigh number),
while no convection will occur ifRa < 1,708.107 The exact value can only be determined by numerical
solution of the fluid flow in the chamber. In the case of horizontal temperature gradients (orthogonal to
gravity), all values of the Rayleigh number lead to convection, and the magnitude of the velocity of the
fluid flow is proportional to the square root of the Rayleigh number.
Density gradients can also arise in the solution due to variations in the concentrations of the
chemical species present in the solution. Variations in the monomer concentration are caused by
depletion of monomer from the solution at the surface of the growing film and in the bulk. Also
generation of dimers, trimers, and other soluble byproducts in the bulk solution may result in additional
concentration density gradients. Such solutal gradients, along with the temperature gradients, can give
rise to double-diffusive convection. This complicated convective motion is usually analyzed with the aid
of the solutal Rayleigh number, in addition to the thermal Rayleigh number.1°6 The solutal Rayleigh
number, Ras, is defined as
Ras _ flgd 3 AC , (5)vD
where fl is the coefficient of concentration expansion; AC is the concentration difference across distance,
d, in the solution; and D is the diffusion coefficient. Double-diffusive convection flows can be far more
complex than simple thermal convection flows.
Hence we see that convection can arise by several means during PDA film photodeposition
from solution. The extent of convection, and its intensity and structure, can only be understood through
accurate numerical modeling of the fluid motion and thermodynamic state of the system.
4. Transport of Particles from Bulk Solution
One significant effect of convection can be seen when PDAMNA films grown in 1-g are viewed
under an optical microscope: they exhibit small particles of solid polymer embedded throughout. These
form when polymer chains in the bulk solution collide due to convection and coalesce into small solid
particles, on the order of a few hundedths of a micron in size. Because these particles are so small,
almost colloidal in nature, they do not sediment out readily, and thus remain suspended in the bulk
solution. Convection then transports these particles to the surface of the growing film where they
become embedded. These particles are defects that can scatter light and thus lower the optical quality of
the films.
32
To studytheeffectsof convection on the occurrence of these particles in the films, the growth
chamber was placed in different orientations with respect to gravity in order to vary the fluid flow
pattern. 108 PDAMNA films were grown with the chamber vertical (irradiating from the top) and with
the chamber horizontal (irradiating from the side). In the case when the chamber is vertical and the
solution is irradiated from the top, the axial temperature gradient is vertical with respect to gravity, and
the bulk solution is stably stratified because warmer, less dense solution is above cooler, more dense
solution. Thus in this orientation, convection should be minimized. In the case when the chamber is
horizontal and the solution is irradiated from the side, the axial temperature gradient is horizontal with
respect to gravity, which makes the density gradients less stable. Hence convection should be much
more pronounced in this orientation. Numerical simulations of the fluid flow are consistent with these
expectations. 108
This is reflected in the distribution of solid particles observed in the PDAMNA films grown
in the two different orientations. Films grown with the chamber horizontal clearly contain a greater
concentration of particles than films grown with the chamber vertical (fig. 19). This is consistent with
expectations based on the relative amounts of convection in the two orientations; films grown under
increased convection contain more particles than those grown under less convection. Also, waveguiding
experiments with these films demonstrate that the films containing more particles exhibit greater light
scattering than those containing fewer particles.
Note that even the film grown in the vertical orientation, where convection is minimized, still
contains particles. Thus while convection is lessened, in this case, it is not eliminated. There are two
reasons for this. First, there are still radial thermal density gradients in the horizontal direction even
when the chamber is vertical because the solution near the side walls is cooler than that near the center;
these can give rise to convection. Also, because the substrate is transparent to UV light, it is not heated
directly by the radiation; the only means by which it receives heat is via conduction. Thus, initially,
there will be some heat flow from the warm solution to the cooler substrate, producing a very shallow
unstable thermal density gradient in the immediate vicinity of the substrate/solution interface, which sits
above the stably stratified bulk solution. Any convection initiated in this unstable layer may penetrate
deeper into the stable layer below, giving rise to the phenomenon of penetrative convection. 1°9 The
bottom line is that even under optimum conditions in l-g, convection is still present during PDA thin
film photodeposition from solution, causing particles in the films.
Not only can convection affect the transport of particles which are polymerized from the bulk
solution, it can also transport colloidal particles which are purposely introduced into the system to alter
the optical properties. The study of the effect of composite geometries on the NLO properties of
materials is an active area of research. One of the most commonly studied systems, discussed earlier,
involves small metal inclusion particles surrounded by a continuous host medium. The linear optical
properties of these composites are described by the theory of Maxwell Garnett. 110 In recent years the
model has been extended to include nonlinear materials. 111 112 Unfortunately, assimilation of metal
particles into highly nonlinear solid-state materials is difficult using traditional chemical techniques
because most highly nonlinear polymeric host candidates require organic solvents which are
ncompatible with the colloid. Ion implantation is a realistic alternative but is not readily available
Public reporting burdenfor this collectionof informationis estimatedto average 1 hourper response, including the timefor reviewinginstructions,searching existing data sources,gatheringand maintainingthe data needed, and compistingand reviewingthe collectionof information. Send commentsregardingthis burdenestimate or any other aspectof thiscollectionof information,includingsuggestionsfor reducing this burden, to WashingtonHeadquartersServices,Directorate for InformationOperation and Reports,1215 JeffersonDavis Highway, Suite 1204, Arlington,VA 22202-4302, and to the Office of Management and Budget,Paperwor_ ReductionProject(0704-0188), Washington, DC 20503
1. AGENCY USE ONLY (Leave Blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED
March 1997 Technical Memorandum
4. TITLE AND SUBTITLE 5. FUNDING NUMBERS
Microgravity Processing and Photonic Applications of Organic
and Polymeric Materials (MSFC Center Director's Discretionary
Fund Final Report, Project No. 95-26)
6. AUTHORS
Donald O. Frazier,* Mark S. Paley,; Benjamin G. Penn,* Hossin A.
Abdeldayem,$ David D. Smith,* and William K. Witherow*
Prepared by Space Sciences Laboratory, Science and Engineering Directorate
*Marshall Space Flight Center $ Universities Space Research Association
12a. DISTRIBUTION/AVAILABILITY STATEMENT
Unclassified-Unlimited
12b. DISTRIBUTION CODE
13. ABSTRACT (Maximum 200 words)
Some of the primary purposes of this work are to study important technologies, particularly involving thin films, relevant to organic
and polymeric materials for improving applicability to optical circuitry and devices and to assess the contribution of convection on film quality
in unit and microgravity environments. Among the most important materials processing techniques of interest in this work are solution-based
and by physical vapor transport, both having proven gravitational and acceleration dependence. In particular, polydiacetylenes (PDA's) and
phthalocyanines (Pc's) are excellent nonlinear optical (NLO) materials with the promise of significantly improved NLO properties through
order and film quality enhancements possible through microgravity processing.
Our approach is to focus research on integrated optical circuits and optoelectrortic devices relevant to solution-based and vapor processes of
interest in the Space Sciences Laboratory at the Marshall Space Flight Center (MSFC). Modification of organic materials is an important aspect
of achieving more highly ordered structures in conjunction with microgravity processing. Parallel activities include characterization of materials
for particular NLO properties and determination of appropriation device designs consistent with selected applications.
One result of this work is the determination, theoretically, that buoyancy-driven convection occurs at low pressures in an ideal gas in a
thermalgradient from source to sink. Subsequent experiment supports the theory. We have also determined theoretically that buoyancy-driven
convection occurs during photodeposition of PDA, an MSFC-patented process for fabricating complex circuits, which is also supported by
experiment. Finally, the discovery of intrinsic optical bistability in metal-free Pc films enables the possibility of the development of logic gatetechnology on the basis of these materials.
14. SUBJECT TERMS
Nonlinear Optics, Organic- and Polymeric-Based Devices, Photonics,
